1. Field of the Invention
The present invention generally relates to vehicle passenger restraint systems, and in particular to a passenger sensing system for detecting the presence and position of passengers within a vehicle.
2. Description of the Prior Art
Since their introduction in the late 1980's airbags have saved lives and reduced the severity of countless injuries resulting from automobile accidents. However, airbags have also taken lives and caused injuries to very young children when the force of a deploying airbag struck a rear-facing child safety seat. In response to these tragic events, the National Transportation Safety Board (NTSB) issued a recommendation to the automotive industry to develop "smart" passenger restraint systems which can detect a rear-facing child safety seat and automatically deactivate adjacent airbags. The present invention is provided in response to the NTSB's recommendation.
Several passenger sensing systems are currently known. A first sensing system em ploys a load-dependant switch is mounted under the seat cushion of a vehicle seat. When the seat is occupied by an object of sufficient weight, the switch is activated to arm the passenger restraint system. However, a disadvantage of this system is that it cannot distinguish between, for example, an occupied child safety seat and a small passenger. Such a system is disclosed in U.S. Pat. No. 3,863,209.
A second passenger sensing system uses optical or ultrasonic transmitters and sensors to detect the position and motion of a passenger. However, the optical and ultrasonic transmitters and sensors are expensive and difficult to install properly within a vehicle cabin. Such a system is disclosed in U.S. Pat. No. 5,446,661.
A third type of passenger sensing system utilizes a pair of electrodes mounted on a vehicle seat to detect a passenger. With this system, the presence or absence of a passenger changes the capacitance between the electrodes. The electrodes are connected to a variable oscillator circuit whose frequency is changed in response to the electrode capacitance. By measuring the frequency of the variable oscillator circuit and comparing the measured frequency with a threshold value, the presence or absence of the passenger is determined. The threshold value is selected such that a passenger restraint system is activated when the detected passenger is larger than a predetermined size, and is de-activated when the detected passenger is smaller than the predetermined size. This capacitive passenger sensing system is disclosed in PCT Application No. WO 95/21752.
A problem with the foregoing variable oscillator passenger sensing system is that airbags are de-activated even in "safe" situations, such as when a forward-facing child safety seat is used.
Further, with this variable oscillator sensing method, the effect of an object on the capacitance between the electrodes is substantially independent of its orientation. Therefore, it is difficult to use this method to distinguish between, for example, a child riding in a front-facing child seat and a child riding in a rear-facing child seat. That is, a child will produce the same variable oscillator frequency whether the child is riding in a rear-facing child safety seat or in a forward-facing child safety seat.
In view of the shortcomings of the prior art passenger sensing systems, what is needed is an economical passenger sensing system and method which can distinguish between a front-facing child seat and a rear-facing child seat.